Radio receiver



G. C. SZIKLAl RADIO RECEIVER Dec. 31, 1946.

Filed Aug. 5, 1944 m R. mm r W m a M W S. M I c T E Mun vfi i QWDQQQET wE bmiw g 6 HE Q95 Y Y llV QR B w! Q SQ 365 H ER QQTE QQEQ E PatentedDec. 31, 1946 RADIO RECEIVER George C. Sziklai, Princeton, N. .L,assignor to Radio Corporation of America, a corporation of DelawareApplication August 5, 1944, Serial No. 548,162

9 Claims. (Cl. 250-27) Y My present invention generally relates toreceivers of radio signals whether angle modulated or amplitudemodulated, and more particularly to an improved detector of frequencymodulated (FM) carrier waves.

In my U. S. Patent No. 2,251,382, granted August 5, 1941, there areshown various arrangements for reducing the effect of carrier amplitudevariations during detection of angle modulated carrier waves, as forexample frequency modulated carrier waves. In the patented circuits aspecial amplitude limiter prior to the discriminator section of thedetector was dispensed with. Variations in carrier amplitude weresubstantially reduced by varying the mutual conductance of the balanceddetector tubes, or of the audio amplifier tube, inversely with thecarrier amplitude thereby to overcome the effect of the amplitudevariations.

One of the important objects of my present invention is' to improve themethod of compeneating for carrier amplitude variation of my aforesaidpatent by changing the internal resistances of the balanced rectifiersproportionally to the carrier amplitude whereby there may be employedgridcontrolled diode rectifiers in the detector circuit-gin place of themulti-grid tubes of my aforesaid patent.

Another important object of my invention is to utilize the rectifierdevice furnishing the compensation voltage during frequency modulationdetection, for supplying modulation voltage in response to reception ofamplitude modulated (AM) carrier waves.

Another important object of my invention is to impart a double functionto arectifier device embodied in a frequency modulation-amplitudemodulation receiver; the rectifier furnishing modulation signals duringamplitude modulation reception, and providing compensation voltage toreduce the efiect of carrier amplitude variation during detection offrequency modulated carrier waves. Another object of my presentinvention is to provide an FM-AM receiver which does not employ aspecial amplitude limiter stage for FM reception while being relativelynon-responsive to carrier amplitude changes, existing tubes of the FMreceiver being employed for AM reception.

Still other objects of my ,invention are to improve generally theoperation of FM receivers. and to provide a compact, efficient andstable FM-AM receiver. V v

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims; the inven- 1tion itself, however, as to both its organization and method ofoperation will best be understood by reference to the followingdescription, taken in connection with the drawing, in which I haveindicated diagrammatically two circuits whereby my invention may becarried into effect.

''-In the drawing:

Fig.1 shows an FM-AM receiving system embodying the invention; and

Fig. 2 is a modification of the demodulator circuit of Fig. 1.

Referring now to the accompanying drawing, there is shown in Fig. 1 somuch of an FM-AM receiver as is essential to a proper understanding ofmy invention. The various objects mentioned above are all achieved inthe system shown. Let it be assumed that tube I is the finalintermediate frequency (I. F.) amplifier of a superheterodyne receiver,adapted to be operated in one or more frequency ranges of the radiocommunication spectrum.

The superheterodyne form of receiver is widely used for FM and ,AMreception. While my present invention is not restricted to any specificfrequency ranges so far as FM or AM reception is concerned, by way ofspecific example it may he assumed that FM reception is had in theasvalue of 4.3 mc..is found most desirable.

signed FM broadcast band of 42-50 megacycle (mc.), while AM .receptionis had in the 550-1700 kilocycle (kc.) AM broadcast band. Those skilledin the-art or radio communication are fully acquainted with multi-rangesuperheterodyne receiver construction-wherein a plurality of frequencyranges are selectively operated for signal reception. Multi-rangesuperheterodyne receivers are provided in the signal selector circuitsand inthe local oscillator circuit with suitable band switching devicesso as to produce a suitable I. F. signal depending upon the frequencyrange of reception. For example, in receiving FM signals in the 42-50mc. band, anoperating I. F. On the other hand when receiving signals inthe AM broadcast band of 550-1700 kc., a usual operating I. Fjvalue is455 he. Hence, there are usually provided in the plate circuit of thecon-. verter tube independent resonant circuits which are respectivelytuned to the FM or AM operating I. F. values. The respective I. F.signal energies are developed in these independent I. F. circuitswithout interference with each other. The independent I. F. circuits areutilized in the input in the plate circuit of a prior I. F. amplifiertube. Furthermore, let it be assumed that circuit 2 is tuned to theoperating 1. F. value of 4.3 me. which is employed when receiving FMsignals, while circuit 3 is tuned to the LF. value of 455 kc. which isemployed when receiving AM signals. The

+B voltage supply line is bypassed to ground for I. F. currentsbycondenser 4 in order to provide return path to the cathode circuit ofthe prior I. F. amplifier tube. Circuit 2 is magnetically coupled toresonant circuit 5, whereas circuit 3 is magnetically coupled toresonant. circuit 6. Cir-- cuits 5 and 6 are arranged in series in thegrid circuit of the I. F. amplifier tube i. Circuits 5 and 6 under theconditions set forth above are tuned respectively to 4.3 mo. and 455 kc.The lower end of circuit 6 is preferably connected to a point. ofnegative direct current potential so as to bias the grid 1 of the triodesection of amplifier tube I., the cathode 8 of the amplifier tube beinggrounded. Condenser 9 provides an intermediate frequency return path toground-for the grid circuitofthe tube 1.

It is to be understood that circuits 2 and 5 are magnetically coupled toprovide a passband width sufiicient to transmit the maximum frequencyvariations of the signal waves. In the 42-50 mo. band the maximumfrequency swing is.150 kc., and it is preferred to" have the passbandsof .the. coupled selector circuits of the FM channel of the receivereach 200 kc. wide. As those skilled in the art of radio communicationl:now,an FM wave has itsfrequency varied in accordance with the.amplitude of the-modulating signal which is'applied at the transmittertransmit the. sideband' components: of. the AM.

waves... In the AM broadcast bandthe permissible channel of transmissionis 10 kc;, but it is preferred to have the circuits 3 and 6. coupled totransmit a band which is. 20. kc. Wide. Similarly in the plate circuitof the triode section of tube I there are arranged in series theresonant circuits it; and H. Circuit iii is tuned to 4.3 mo... whilecircuit H is. tunedto4'55 kc. The low potential side of circuit H isreturned to ground by condenser 12' for I. F. currents. A selective.means is provided with each of the. series-connected resonant circuits.for selectively short-circuiting' circuits 3; 6 and H' respectively.These circuits are shcrt-circuited. Whenit isdesired to receive FMsignals. Thus, adjustable .switch arms. I3, I14" and I5 are connected tothe, low potential sides Qfc'ircuits. 3, 6 audit respectively. Eachofthe switch. arms 13., Mend 15 has. associated therewithlan. AM contact.The FM contact for each. of. the switches. is. connected to the junctionof eachrespective pair of. series-connectedresonant circuits.Accordingly, the. receiver. is adjustcd. for FM- reception when. eachof, switch arms is... it and I5. isadiustedto its. respective FM contactthereby short-circuiting the. re spective. circuits; 3,. E. and H.However, when eachof these-switch arms is adjusted. to the free.

AM contacts then the receiver is conditionedto receive AM signals, sinceeach of circuits 2, 5 and ID has no effect on the associated circuits 3,E and I i at the lower I. F. value of 455 kc. This follows from the factthat the capacity elements of respective circuits 2, 5 and II] have verylow impedance at the lower LF. value of 455 kc. It is to be clearlyunderstood that the various selector circuits preceding circuits 2 and3, and not shown herein to preserve simplicity of description, will beconstructed in the manner describedherein.

The tube 1 is provided with an auxiliary diode anode Eli which islocated adjacent cathode 8,, but out of the main electron stream toplate 2 l Tube i may-be of the 63K? type. Cathode 8 and anode 20.provide a diode rectifier which performs a double function in thereceiving system. These dual functions will be described at a laterpoint. While I have shown the tube i' as embodying a diode section,those skilled in the art will realize that a separate diode tube couldbe used instead. The anode is connected by lead 22 to the midpoint ofcoil 23. The latter coil is actually the secondary winding of thetransformer M whose primary circuit in is tunedto the operating I. F;value of 4.3 me. The coil 23 is shunted by condenser 24 and tunes theformer to the 4.3 mc. frequency'value. The coupling between primarycircuit Ill and secondary circuit 23. 24 is such that a band passresponse having a 200 kc. width is provided.

The plate side of circuit H3 is connected to the phase of the I. F.currents-passed therethrough.

The condenser functions essentiallyas a direct current blockingcondenser and acts asa direct connection for FM currents from the highalternating potential side of circuit Hi to the midpoint of coil 23.There is thus provided a method of translating applied FM voltage into apair of AM voltages of the same frequency but Whose relative magnitudesare a function of the sense and extent of frequencydeviation ofthe FMsignal waves. The frequency discriminator shown is that described andclaimed by S. W. Seeley in his U. S. Patent No. 2,121,103, granted June-21, 1938. The present invention is not restricted to that type offrequency discriminator, as there may be employed in place of circuit23, 24 a pair of resonant circuits tuned to frequencies located onopposite sides of the frequency of input circuit it; That form ofdiscriminator is shown by G. L. Usselman in U. S. Patent No. 1,794,932,grantedMarch 3, 1931. In general, then, my invention is independent ofthe particular construction of the frequency discriminator con nected tothe pair of detector tubes 26 and Z'L Each-of detector tubes 25 and 21is shown as a triode; they are used specifically as gridcontrolled dioderectifiers. The tubes 26 and 21 may each be of the 6SL7 type, but theinvention is in no way restricted to any specific types of tubes. Itwill be seen that except for the internal resistance control of tubes26-and 21, the latter function as if. they were diodes arranged inopposed relation. This is readily observed from th fact that anodes 28and 29 of respective tubes Ziiand 21 are. connected to opposite ends ofcoil 23,,while cathodesfiil and 3| are respectively-connected toopposite ends of output load resistors 32 and 33. The. midpointof coil23.is connected by I..F.. choke coil 34 tothe junction. of loadresistors.32.and 33.

Each of load resistors 32 and 33 is shunted by Y a respective I. F.bypass condenser and 36. The upper end of load resistor 32 is connectedthrough the modulation signal coupling condenser 3'! to the output lead38 terminating in the contact FM. The internal resistance between theanode and cathode of each of tubes 26 and 21 is regulated automaticallyin dependence on the AM effects which may exist on the FM carrier waveenergy at output circuit Hi. In other words, the internal resistances ofthe opposed rectifiers are changed proportionally to the FM carrieramplitude variations. The control is secured by providing control gridsand 4| in the respective space current paths of tubes 26 and 21. Thegrids 43 and 4| are conductively connected to slider 42 arranged to beadjusted along resistor 43 shunted by condenser 44.

The resistor 43 is arranged in the space current path of diode section8, 20 by connecting the upper or ungrounded end of resistor 43 to anode20 through leads and 22. Since cathode 8 of tube I is grounded, it willbe seen that resistor 43 is traversed by the rectified signal cur rents.The condenser 25 applies the amplified I. F. signal currents to anode20. The condenser 44 is chosen to bypass I. F. currents, but notmodulation components. In other words, the constants of condenser 44 andresistor 43 should be so chosen that network 43, 44 has a relativelyshort, i. e., fast, time constant. The time constant should be capableof permitting the highest modulating signal frequency (say about 15,000cycles) expected to be encountered to develop a voltage across resistor43.

Hence, the voltage applied to grids 43 and 4! is proportional to AMcomponents thatmay exist on the FM carrier wave. Such AM components arecreated as a result of diverse effects. Even though an FM wave istransmitted from the transmitter as a constant-amplitude wave, yet bythe time it is applied to the frequency discriminator it may well haveacquired AM components. Special amplitude limiters have been employedprior to the discriminator. These, while effective, require a highsignal gain at the limiter tube input electrode. In the case of portableor compact radio sets, for example those of the battery-operated typewhere space is at a premium, it is virtually impossible to attain therelatively high gain needed for driving a limiter tube.

Hence, in my present system there is no special limiter tube employed.Instead the control. grids 40 and 4! are provided in the usual FMdetector diodes, and the grids have applied thereto control voltagederived from a rectifier embodied in an existing I. F. amplifier tube.No extra tube has been added to the FM receiver, and yet any AMcomponents existing on the FM signals at circuit H) are instantlyrectified and caused to bias grids 40 and 4! in a sense to increase theinternal resistance of each detector tube.

Furthermore, rectifier 8, 23 is caused to per form an additionalfunction. Upon adjustment of the receiver for AM carrier wave reception(arms l3, l4 and i5 adjusted to AM contacts), amplified AM wavesdeveloped at circuit ll will be applied through condenser 25 to therectifier 8, 261. The rectified voltage appearing across resistor 43 inthat case corresponds to the ampli tude modulation components existingon the desired AM carrier wave. Such modulation is utilized byconnecting the AM contact of switch arm to the upper end of resistor 43through coupling condenser 5|. Switch arm 50, of course,

iii

may be connected to any desired audio frequency amplifier network (notshown).

Considering, now, the operation of the FM-AM system in detail, andassuming first that all reception switches are adjusted to FM position,there will be developed across circuit I 0 signals whose centerfrequency is 4.3 mc. The various circuits 3, 6, l l have beenshort-circuited, and switch arm 50 is on contact FM. Due to the magneticcoupling between the primary and secondary circuits l0 and 23, 24 therewill be induced in each half of coil 23 a signal voltage in phasequadrature with the primary signal voltage. However, the inducedvoltages are of opposite polarity due to the efiective grounding of themidpoint of coil 23. The midpoint is, also, varied in potential in thesame manner as the plate side of primary circuit Hi. This directlyinjected voltage appears across each half of coil 23 in like polarity.Hence, at the anodes 28 and 29 appear the respective resultant voltagesacross each half of coil 23, and these resultants are the vectorresultants of the quadrature-related induced and directly-injectedvoltages across each half of coil 23. When the I. F. signals are at thecenter or reference frequency of 4.3 mc., the vector resultant voltagesat anodes 28 and 29 are equal. However, as the frequency of the signalenergy across circuit l0 deviates from 4.3 mc., the resultant voltagesbecome unequal in the direction of frequency deviation due to the factthat the aforesaid quadrature relation exists only at the instant whenthe signal energy frequency is equal to the reference frequency of 4.3mc.

As is explained in the aforesaid Seeley patent, the variable-amplitudesignal voltages at the anodes 28 and 29 are rectified by the respectivediode rectifiers 28, 30 and 29, 3|. The rectified voltage output ofrectifier 28, 30 appears across load resistor 32, while the rectifiedvoltage out put of the other rectifier appears across load resistor 33.Since the cathode end of resistor 33 is grounded, there will be takenoff from the cathode end of resistor 32 the differential resultantvoltage of the voltages across resistors 32 and 33. At the instant whenthe I. F. signals are 4.3 mc., the voltages across resistors 32 and 33will be equal and the modulation output will be zero. The rapidfrequency variations or deviations, corresponding to the modulatingsignals applied to the carrier at the transmitter, will. appear as amodulation voltage output at the cathode end of resistor 32 as describedin Seeley Patent 2,121,103 and as is well known in the art.

Should there suddenly appear an undesired AM component on the carrier,such undesired amplitude modulation of the carrier will appear as avariable voltage across resistor 43. It is to be understood that duringthe normal FM reception there will be a rectified carrier voltage acrossresistor 43. Of course, as long as the carrier amplitude is uniform orconstant the grids 4! and 43 will have substantially constantpotentials, which will establish the normal internal resistance existingbetween the cathode and anode of each of tubes 26 and 21. As a matter offact the slider 42 may be adjusted to a point on resistor 43 so as toprovide a minimum value of internal resistance for each of the detectortubes. Having set slider 42 to a minimum internal resistance point, anincrease in carrier amplitude will resultin a variation of the potentialof each of grids 40 and 4|.

It will be noted that the internal resistance of ondary circuit 23,:24'asshown in Fig. 1.

grids 10 and ii will become proportionally more negative. Consequentlythe internal resistance of each of the detector tubes will be increased,and this increase will be proportional to the increase of the FM carrierwave amplitude. Since the internal resistance of each of the detectortubes has been increased, any increased voltage impressed upon diodes23, 30 and 29,- 3! will not be reflected across load resistors 32 and33, but will be compensated for by virtue of the fact that the internalresistance of each of tubes 26 and 21 has been increased. In this wayamplitude modulation of the FM carrier wave will be very largelycounteracted at the detector tubes 25 and 21 so as to prevent anyundesired increase in rectified voltage across each of resistors 32 and3.3.

If, now, it is desired to receive AM carrier waves it is only necessaryto switch the various band switching devices of the receiver, and moreparticularly switch arms i3, i4, i5 and 50 are adjusted to the AMcontacts. When the receiver is thus conditioned, the circuits 3, 6 and Hwill have developed across each of them the I. F. signal energy at 455kc., while the circuits 2, 5, l0 and 23, 2'4 will be ineffective at thelower I. F. value. The tube i will function as an I. F. amplifier,whereas tubes 26 and 2'! will be ineffective. The diode rectifier 8, 29will have applied thereto the I. F. signal energy of 455 kc. by virtueof the condenser 25. It is to be understood that the resonant circuit IIwill be given a response curve such as to pass eiiiciently the entire I.F. signal energy. There will be produced across resistor 13 themodulation signal voltage representative of the AM components of the AMcarrier wave energy. These modulation signal components are taken ofithrough coupling condenser 51 and switch 50, and utilized in the commonaudio fre quency amplifier network.

It will, therefore, be seen that I have provided a compact and highlyeconomical circuit arrangement for receiving either FM or AM car rierwaves, and in the FM- reception condition use of AM limiting is madeunnecessary by the provision of means for varying the internalresistances of the rectifier tubes in a manner to compensate for theexistence of any AM components on the received FM waves. It will beobserved that there is no need for a special limiter tube prior to thefrequency discriminator network, and that the device which provides thecompensating voltage for the detector tubes is included within the lastI. F. amplifier tube. It is to be clearly understood, however, that thediode 8, may be located outside the amplifier tube i. Furthermore, myinvention is not restricted to the utilization of independenttransmission channels for FM and AM signal waves, since a commontransmission channel may be employed in the receiver where the FM and AMsignal waves lie in a common range of frequencies.

In Fig. 2 I have shown a modification of the system wherein the I. F.amplifier i has an output circuit ii! magnetically. coupled to. the secamatter of fact the system shown in Fig. 2 is substantially similar tothat shown for the FM transmission channel of Fig. 1, and for thisreason identical circuit elements are indicated by the same referencenumerals, except that in the case of Fig. 2 a prime designation isemployed. Furthermore, in Fig. 2 the detector electrodes are shown asembodied in a common tube envelope 26'; It will be understood that inFig. 2 there may be developed across resonant circuit iii FM or AMsignal waves having a carrier frequency which will be equal to theoperating I. F. value of 4.3 mc. Hence, the secondary circuit 23', 24'will be tuned to the same I.'F. value.

For FM reception it is only necessary to switch arm 5!! to the FMcontact which connects through condenser 31' to the cathode end ofresistor 32'. In this position of the switch arm 5!! the frequencydiscriminator input network produces the hereinbefore describedresultant vector voltages at anodes 28 and 29', and the differentialrectified voltage is produced across load resistors 3?." and 33 inseries. The diode 26'', whose cathode 8' is connected to the outputterminal of condenser 25 and whose anode 20' is connected to theungrounded end of resistor 53, provides the compensating voltage. Itwill be noted that in this case the diode 29" has its cathode 8'returned to ground through a path comprising lead 45, choke coil 34. andresistor 33'. Undesired amplitude modulationinthe FM carrier wave iscompensated for by applying rectified voltage across resistor 43'togrids M and 2-21. The functioning of the compensating circuit in Fig. 2is precisely the same as inthe case of Fig. 1. To receive AM signalwaves, switch arm 59 is adjusted to the AM contact. In this case the AMsignal waves, also at an I. F. value of 4.3 me, are applied to therectifier 28" through the coupling condenser 25'. The modulation signalvoltage across resistor. 43' is utilized by the following audiofrequency amplifier. Of course, the circuits i0 and 23, 25' may be tunedto any common desired I. F.'value. Furthermore, in the modification ofFig. 2 the diode 2G" may have its electrodes included as a part of theI. F. amplifier tube i in the manner shown in Fig. 1, if desired.

While I have indicated and described several systems for carrying myinvention into efiect, it will be apparent to one skilled in theartzthat my invention is by no means limited to the particularorganizations shown and described, but that many modifications may bemade without departing from the scope of my invention.

What I claim is:

1. In combination in a frequency modulated wave receiver of the typecomprising a demodulator having an input circuit upon which ,isimpressed frequency modulated waves,.said .demodulator consisting of apair of trio-des connected as diode rectifiers in a push-pull circuitconstructed and arranged to convert the waves into modulation voltage;the improvement which comprises means coupled to said input circuit forproducing a uni-directional voltage in response to amplitude variationsin the impressed waves, and means responsive to said uni-olirectionalvoltage for controlling the internal resistance between the cathode andanode of each diode rectifier in like senses and to'an extent tocompensate for said amplitude variations.

. 2. In combination, a frequency-modulator detector system of the typeutilizing a pair of opposed diode rectifiers, a signal amplifierincluding a diode rectifier section, said amplifier being provided withan output circuit coupled to said rectifiers, means for applyingfrequency modulated carrier wave energy developed in said amplifieroutput circuit to said rectifier section, the improvement comprisingmeans. responsive to rectified voltage developed from rectified currentsof said diode section, and said last means for varying the internalresistance of each of said opposed diode rectifiers in like senses.

3. In a combined frequency modulation and amplitude modulation receiver,a signal amplifier tube provided with an amplifier section and a dioderectifier section, separate signal input channels connected to saidamplified section inut electrodes tuned to diiferent operatingfrequencies for respective frequency modulation and amplitude modulationreception, a similar pair of transmission circuits connected to theoutput electrodes of said amplifier section, a pair of independentgrid-controlled diode rectifiers connected in a push-pull circuit andhaving an input circuit coupled to said amplifier output circuit whichis tuned to the frequency modulation signals, means for applyingfrequency modulated signal waves developed in the frequency modulationoutput circuit of the amplifier section to said diode section, means forderiving a control voltage from said diode section, and additional meansfor automatically varying the grid potential of each of said opposedrectifiers thereby to vary the internal resistanoe of the latter in asense to compensate for any amplitude variations which exist onreoperatively associated with each rectifier, and

prcvement which comprises means coupled to Y input circuit for producinga uni-directional voltage in response to amplitude variations in theimpressed Waves, and means responsive to said uni-directional voltagefor controlling the internal resistance between the cathode and anode ofeach diode rectifier in like senses.

5. In combination with a frequency modulator detector system of the typeutilizing a pair of triodes connected as opposed diode rectifiers, asignal amplifier including a diode rectifier section, means coupling theamplifier output circuit to said opposed rectifiers, means for applyingfrequency modulated carrier wave energy developed in said amplifieroutput circuit to said diode rectifier section, means responsive torectified voltage developed from rectified currents of said diodesection, and said last means varying the internal resistance of each ofsaid opposed diode rectifiers in like senses.

6. In an angle modulated wave receiver of the type comprising ademodulator having an input circuit upon which are impressed anglemodulated Waves, said demodulator consisting of a pair of dioderectifiers connected in a pushpull circuit constructed and arranged toconvert the waves into modulation voltage; the improvement whichcomprises rectifier means coupled to said input circuit for producing aunidirectional voltage in response to amplitude variations in theimpressed waves, a control grid means responsive to said uni-directionalvoltage and connected to each control grid for control- -ling theinternal resistance between the cathode and anode of each dioderectifier in like senses and to an extent to compensate for saidamplitude variations.

'7. In combination in a frequency modulated wave receiver of the typecomprising a demodulator having an input circuit upon which is impressedfrequency modulated waves, said demodulator consisting of a pair oftriodes connected as diode rectifiers in a push-pull circuit constructedand arranged to convert the waves into modulation voltage, means toutilize said modulation voltage; the improvement Which comprisesrectifier means coupled to said input circuit for producing auni-directional voltage in response to amplitude variations in theimpressed. waves, means responsive to said unidirectional voltage forcontrolling the respective control grids'of said triodes in likepolarity senses thereby to vary the internal resistance cetween thecathode and anode of each diode rectifier in like senses and to anextent to compensate for said amplitude variations, and means forselectively switching the connection of the utilizing means from saiddemodulator to said rectifier means for reception of amplitude modulatedwaves.

8. In a combined frequency modulation and amplitude modulation receiver,a signal amplifier tube provided with input and output electrodes, arectifier, separate signal input circuits tuned to different operatingfrequencies for respective frequency modulation and amplitude modulationreception, means connecting the input circuits between said amplifierinput electrodes,.a similarly tuned pair of amplifier output c cultsconnected between the output electrodes of said amplifier, a pair ofindependent grid-controlled diode rectifiers connected in a push-pullcircuit and having an input circuit coupled to the amplifier outputcircuit of said pair which is tuned to the frequency of frequencymodulation signals, means for applying frequency modulated signal Wavesdeveloped in said last-named amplifier output circuit to said rectifier,means for deriving a control voltage from said rectifier, additionalmeans for automatically varying the grid potential of each of saidopposed rectifiers thereby to vary the in ternal resistance of thelatter in a sense to compensate for any amplitude variations which existon received frequency modulation signals.

9. In a frequency modulated Wave receiver of the type comprising ademodulator having an input circuit upon which are impressed frequencymodulated Waves, said demodulator consisting of a pair of dioderectifiers, connected in a push-pull circuit constructed and arranged toconvert the waves into modulation voltage; the improvement whichcomprises a control rectifier coupled to said input circuit forproducing a uni-directional voltage in response to amplitude variationsin the impressed waves, means responsive to said uni-directional voltagefor con-trolling the internal resistance between the cathode and anodeof each diode rectifier in like senses, and means for selectivelyderiving modulation signals from said control rectifier in response toamplitude modulated carrier waves being applied to said input circuit.

GEORGE C. SZIKLAI.

